A. Technical Field
The present invention relates to production processes for a basic water-absorbent resin and a water-absorbing agent, and use thereof.
More particularly, the present invention relates to a production process for a water-absorbing agent which exhibits a greatly enhanced water absorption capacity under a load, and a production process for a basic water-absorbent resin that is a main component thereof.
B. Background Art
In recent years, so-called water-absorbent resins are widely used as constituent materials of sanitary materials, such as disposable diapers, sanitary napkins, and so-called incontinent pads, for the purpose of causing the water-absorbent resins to absorb body fluids.
Known examples of the above water-absorbent resins include: crosslinked products of partially neutralized polyacrylic acids; hydrolyzed products of starch-acrylic acid graft polymers; saponified products of vinyl acetate-acrylic acid ester copolymers; and hydrolyzed products of acrylonitrile- or acrylamide copolymers, and their crosslinked products.
However, almost all of these conventionally known water-absorbent resins are products by neutralizing their acid groups of acidic water-absorbent resins with compounds of alkaline metals. Therefore, in the case where the surface neighborhood of particles of the resins is not secondarily crosslinked, their absorption capacity under a load for a physiological salt solution is below 10 g/g, and further, even if the surface neighborhood of particles of the resins is secondarily crosslinked, their absorption capacity under a load for artificial urine is merely 30 g/g at the highest. Thus, it is desired to develop a novel water-absorbing agent which can absorb much water to swell therewith even under loaded conditions.
To solve such problems, WO 96/15180, WO 96/17681, WO 98/24832, WO 98/37149, WO 99/34841, WO 99/34842, WO 99/34843, WO 99/25393, and WO 99/30751 propose water-absorbing agents which comprise an acidic water-absorbent resin and a basic water-absorbent resin. However, in the methods described in these patents, the basic water-absorbent resin to be a main component of the water-absorbing agents was pulverized once after being in a lump gel state, and blended with the acidic water-absorbent resin further after adjusting the particle size of the basic water-absorbent resin to the most suitable size if necessary. Therefore, in large scale production, very large lump gel has to be treated, and the equipment therefor not only becomes large but also has low productivity. In addition, batchwise qualities could not but be unstable. Particularly, if the hydrogel was pulverized mechanically after forming the lump gel once, networks once formed by crosslinking had to be cut. Then, there were problems such that an extractable content increased, a crosslinked structure did not become uniform, and much energy was required in order to pulverize the gel. Furthermore, EP 949290A2 proposes a method which comprises carrying out a crosslinking reaction of a basic resin in a suspension state. However, an organic solvent is used in the method. Therefore, the equipment therefor not only becomes large but also has low productivity because the method needs a step of removing the organic solvent.